CA2275939A1 - Radioelectric transponder equipped with frequency detuning antenna and circuit - Google Patents

Abstract

The invention concerns radio transponders (P0, P1, P2) capable of being detected and/or interrogated by an adapted radio system (E). The invention is characterised in that the antenna can be detuned in frequency or maladjusted in impedance such that the transponder (P1) absorbs less radio field (B) and energy and that another transponder (P2) located nearby receives enough radio field (B') and energy to operate properly.

Description

RADIO TRANSPONDER PROVIDED WITH AN ANTENNA AND A
FREQUENCY DISAGREEMENT CIRCUIT.
The present invention relates to the field of detection and / or interrogation of transponders using a detection system and / or adapted radioelectric reading. Such transponders are used particularly for detecting or identify moving objects on which the transponders are arranged.
Tels systems can ~ be used in particular for the recognition of individuals wearing badges, i0 of vehicles carrying badges at the toll of a highway or even my ~~ stored candies or offered for sale in stores.
A transponder is a transceiver (transceiver) automatically responding to external signal from a transmitter of the detection system.
Radio transponders, called badges for some applications, or labels, also have role of transmitting information remotely in response to signals from a system transmitter question mark.
In the following description, the term will be abbreviated in its common form "radio".
Detection and / or interrogation systems for known radio transponders have a transceiver E and transponders T1, T2 such shown schematically in Figure I.
The transmitter / receiver E generates a radio field B (or electromagnetic field). When a T1 transponder is 3o finds within the limits of field B, the

2 transponder. picks up the field and signals its presence to the transmitter / receiver E.
We commonly use so-called transponders "active" which thus re-emit a radio signal called S response signal intended for the transmitter / receiver E. The response signal may include information to identify the transponder and / or the object on which it is placed.
To re-transmit a radio signal, the transponders l0 assets may include an autonomous energy source.
But preferably, the transponder recovers the energy radio field B to supply its circuit electronic. To fully capture the field or radio signal, the transponder has an antenna 15 radio. The antenna consists for example of a metal turn of the printed circuit, which the advantage of reducing the dimensions of the transponder.
Some so-called "passive" transponders include just a shorted metal loop. The 2o presence of such a transponder in the field of a transmitter E, constituted as shown in FIG. 1, of a loop traversed by a current, modifies the induction reciprocal of the transmitter loop E and the loop of the T1 transponder.
25 This passive transmitter and transponder system is therefore analogous to the primary and secondary circuit of a Electric transformer. The transmitter / receiver E can thus detect the presence of a T1 transponder in its field by detecting a change in its current 3o of induction.
In addition there are transmission systems â
transponder, in which a radio transmitter sends information that can be saved on a chip electronic contained in the transponder.

3 In general, this request covers all detection and / or interrogation system for transponders, transponders: which can be detected or interrogated by the transmitter / receiver or even saved. The term transmitter / receiver designates generally any device making it possible to emit a radio field and detect or interrogate transponders present in the field, the device preferably being able to receive a radio signal from response from one of the transponders.
Difficulties arise when several transponders appear simultaneously in the field radio B. The transponders then all respond at the same time time at excitation of field B or it does not give reply.
To avoid this confusion, it was proposed to delay the transponder responses, each transponder responding to signal excitation at initialization of the transmitter, after a time random.
However, in many cases, transponders that don't respond to the excitement of the radio field B or radio signal, transponders seeming to be inhibited.
This unresolved problem is mentioned in particular in document FR-A-2 717 593. We note the fact that "two neighboring labels can be found in the field of the same reader, which may cause their automatic mutual inhibition ".
An object of the invention: ion is to optimize the detection and / or inter: rogation of multiples transponders present in the detector field and / or E. readers

4 Another object is to avoid disturbances of the transponder operation caused by proximity other transponders.
An explanation for the reciprocal inhibition of S transponders is that a transponder causes an effect of visible radio shadow in Figure 1, so that another transponder in the shadows no longer receives enough field or energy to operate correctly.
According to the invention, these objects are achieved by providing that the antenna of a transponder can be detuned in frequency or mismatched in impedance, so that the transponder and its electronic circuit absorb less radio field and energy.
Another transponder located near the mismatched or detuned transponder can receive enough radio field and energy to working properly.
The transmission system can then detect or 2o consult this other transponder as if it were alone in field B of the transmitter.
Preferably, according to the invention, there is provided a transponder likely to be presented in a field radio frequency, the transponder comprising an antenna capable of receiving the determined frequency radio field and a electronic circuit coupled to the antenna, the circuit absorbing and restoring energy supplied by the field received by the antenna, characterized in that the 3o transponder includes means for adapting to a reception of a radio frequency field determined and means of adaptation to the reception of the radio field of the determined frequency, the means of adaptation being implemented when the transponder is in a selected state, 3t, the means of maladjustment being implemented when the transponder is in an unselected state in order to

5 limit the absorption of energy and / or field by the transponder in the unselected state.
Preferably, the invention is carried out in providing that the means of adaptation allow to tune the antenna according to the radio frequency l0 determined, and that the means of disadaptation allow you to detune the antenna in relation to the determined radio frequency.
A first embodiment provides that the means of disadaptation and / oL ~ adaptation consist of modify a capacity value, the antenna having a inductance value, so the tuning frequency of the antenna is modified p ~~ r compared to the frequency determined radio signals.
A second mode of reaction: Lization provides that the means of adaptation and / or disadaptation allow to modify the adaptation of im, pedance between the antenna and the electronic circuit.
In an example of the second embodiment, the means of adaptation and / or disadaptation consist of modify a load ~ and / or resistance value in order to modify the antenna impedance value and / or the impedance value of the electronic circuit.
So according to one characteristic, when two transponders are presented in a field radio, each of the transponders receives the radio field and / or frequency signals determined whatever the position of a transponder knows

6 relative to the other transponder, each transponder comprising the means of adaptation and disadaptation to avoid radio interference caused by the proximity of the other transponder.
According to another characteristic, the transponders go into a selected state in turn, only one transponder or a small number of transponders being in a selected state at an instant of time.
According to another characteristic, a transponder lo goes into an unselected state for a period of time corresponding to a timed inhibition of the transponder.
There are also plans for transmissions adapted to transponders according to ts the invention.
The invention will be better understood on reading the description and drawings which will follow, given to title of non-limiting example; on the drawings attached.
20 - Figure 1 shows a block diagram of transponder transmission system according to the state of the technique ;
- Figure 2 shows a block diagram of transponder transmission system according to The invention;
- Figure 3 shows a circuit diagram of transponder according to a first embodiment of the invention;
- Figure 4 shows a circuit diagram of 3o transponder according to a second embodiment of the invention.
Figure 1 shows two transponders T1 and T2 classics presented in field B of a

7 transmitter / receiver E. The T1 transponder, in view direct from transmitter E correctly receives field B.
The radio field provides it with energy to operate and signals ~ r its presence to the transmitter / receiver E and possibly him transmit radio signals in response. In the arrangement of Figure 1, the T2 transponder comes between the transmitter E and the transponder T2 which then hardly receives the radio field B '.
The radio field B and its energy are indeed absorbed by the transponder T1 which forms in some sort of a radio screen. This radio shadow effect prevents often a good detection of the T2 transponder, Ci not receiving enough radio field and energy is to work properly.
we generally observe a mitigation effect or mutual inhibition when the transponders classics are close, so that the transponders are not properly detected or cannot 2o communicate a response to the ems ~ tteur / receiver.
Conventional transponders preferably having reduced dimensions the specialist draws up particularly ac <: antennas ordered at the frequency of radio field that the antenna must receive and perfectly 25 adapted in impedance to ~ s: electronic circuits served by the antenna.
Surprisingly, the invention provides means for mismatching the antenna of a transponder to limit the absorption of the radio field by the 30 transponder when it is not: ~ t not selected. A
neighboring transponder can thus correctly receive the radio field and be detected or communicate with the transmitter / receiver.

8 On the other hand, means of adaptation of the antenna this neighboring transponder are provided for it to receive the radio field.
In Figure 2, similar to Figure 1, we see thus two transponders P1 and P2 arranged in the radio field B emitted by the transmitter / receiver E.
We consider in the example of Figure 2 that the P1 transponder antenna is mismatched, it is â
say that the antenna absorbs little, if at all l0 radio field B. At the level of the second transponder P2, the radio field B 'will then have a normal power as if the P1 transponder was missing. The P2 transponder receives enough energy to operate and be detected or responds to i5 transmitter / receiver E.
The effect of radio shadows or radio screens caused by state of the art transponders is so deleted or mitigated (compare to Figure 1).
A first embodiment of the means of 20 mismatching the antenna of a transponder consists of detune the antenna in relation to the frequency of the radio field of the transmitter, this determined frequency being noted fe.
The antenna of a transponder is generally 25 consisting of a resonant circuit comprising a inductance and a capacity as illustrated in figure 3.
The inductance L can thus be produced in the form of a winding of helical turns. Capacity C can be made up of a single capacitor. She is from 3o preferably made up of a set of capacitors C1, C2 connected in parallel so that the capacity of each of the capacitors C1, C2 of the set added.

9 The well-known frequency of resonance of such circuit is determined by the product of the values with inductance L and capacity C.
One way of mismatching the antenna is to tune the antenna to a resonance frequency fd different from the frequency fe of the radio field or radio signals transmitted to the t: ransponder. In this way the transponder antenna is out of tune with respect to the frequency fe transmitted. The transponder and its antenna then very weakly absorb the field or the radio signal from the transmitter.
Conversely, the invention provides means adapting the antenna of a transponder. These means adapters allow the transponder to receive the radio field or signal in the event of need, especially when the transponder is selected. The first embodiment provides adapt the antenna by tuning the frequency of resonance of the antenna on the frequency fe of the field or 2o of the transmitted signals.
As illustrated in Figure 3, the tuning means of the resonant frequency of the antenna advantageously a modifiable capacity. The capacity modifiable is obtained according to the example of figure 3 with a capacitor V1 connected via a switch I1 in parallel: Lële to capacitors C1 and C2. When switch I1 is closed, the capacity C of the resonant circuit is therefore:
C = C1 + C2 + V1 where C1, C2, V1 are the capacity values of the capacitors CI, C ?. and V1 respectively.
The resonant frequency ~ e fa of the adapted antenna is therefore determined by the following product:
LC = L. (C1 + C2 + V1) WO 98! 29760 PCT / FR97 / 02421 The frequency fa of the adapted antenna will thus be different from the frequency fd of the mismatched antenna, determined by the following product:
LC = L. (C1 + C2) 5 The frequency fd will be correctly shifted by relative to the frequency fa by choosing appropriately the value of capacities C1, C2 and V1.
For example, if the frequency fe of the field or transmitted radio signal is 130 kHz, the antenna will lo tuned to the frequency fa of 230 kHz by choosing the following values of these components:
- one inductance L of 1 ~ CH
- identical capacitors C1, C2, V1 of 0.5 ~ CF.
The product of the inductance and capacitance values is worth so L. (CI + C2 + V1) - 1.5. 10-6 s2, This product corresponds to a frequency of resonance 130 kHz.
The antenna mismatch means may 2o consist, simply, in disconnecting the capacitor V1 by opening the switch I1. The product of values inductance and capacitance of the mismatched antenna is then worth:
L. (C1 + C2) = 1.10-6 s2.
This product corresponds to a frequency fd of resonance of the mismatched antenna of approximately 160 kHz.
The antenna being tuned to a remote frequency fd of the frequency fa of the field or of the signals transmitted by the transmitter it will weakly receive this transmission and will not interfere with receiving the field or signals by a neighboring transponder.
Figure 2 illustrates this situation in which two transponders T1, T2 are in the field radio of transmitter E. Considering that the 1: 1 transponder P1 is in a detuned state, we see that field B is correctly transmitted to the P2 transponder, well P1 transponder interposes.
Equivalently, the antenna agreement may be obtained when the I1 switch is open, the inductance values L and capacitors C1 and C2 then corresponding to the frequency fe of the transmitter.
The antenna is detuned by closing the switch I1 io i.e. by connecting the capacitor V1 or else by connecting several capacitors V1, V2 by through several switches I1, I2.
It is preferable that the resonance frequency fd of the mismatched antenna is close to the frequency of transmitter fe and therefore of the frequency fa of resonance of the adapted antenna. Indeed the unsuitable antenna then weakly receives the radio field or signal. The transponder then has a low energy but sufficient to activate the adaptation means, 2o like the switch I1, and go into a suitable state.
This effect can be advantageously used to activate the transponders in turn as we will see thereafter.
A second embodiment of the means of mismatching the antenna of a transponder consists of perform an impedance mismatch between the antenna and the electronic circuits: ~ D of the transponder.
The antenna of a transponder has indeed a certain impedance at the frequency of the field or 3o transmitted signals.
So that the electronic circuit D served by the antenna receives enough signal and energy, an impedance adaptation is conventionally carried out, that is to say that the electronic circuit is calculated and set to present an input impedance substantially identical to the output impedance of the antenna, for electronic signals having the frequency fe determined.
Figure 4 provides a better understanding of this second embodiment. She shows a transponder similar to that of FIG. 3 in which a resistor R is connectable in the antenna circuit to perform an impedance mismatch. The antenna always has an inductance L and capacitors C1, C2, the capacitance of the capacitors being fixed in this example of a second embodiment. Frequency antenna resonance determined by the product inductance and capacitance values (L. C) is therefore fixed and corresponds to the frequency fe of the field and transmitted signals. In this second embodiment there there is therefore no frequency disagreement. A
switch I allows to connect resistor R in parallel to the components L, C1, C2 of the antenna and to electronic circuit D.
The resistance value R is chosen to modify the antenna impedance value or the value impedance of electronic circuit D, the value of resistance R being preferably low. The value of resistance R may in the extreme be zero, the means of mismatch then returning to short-circuit the antenna, the transponder neither absorbing nor energy, no field.
In operation, the antenna normally receives the field or frequency signals fe.
When switch I is closed, resistance R
changes the impedance of the antenna L, C1, C2 and the antenna is mismatched in impedance compared to the circuit electronic.

In such a mismatched state, the signals electronics and energy received by the antenna are weakly transmitted to the electronic circuit. The antenna of the transponder then absorbs little or not at all the field or transmitted signals ..
In the adapted state if this second mode of realization, switch I is open and the resistance R is disconnected from the circuit. The antenna L, C1, C2 then presents an impedance adapted to l0 the impedance of the electronic circuit D served by the antenna. The antenna then fully transmits the signal and the energy that ~: it receives at the circuit electronic D. The transponder therefore absorbs field B
and energy. I1 can thus be correctly detected by the transmitter / receiver E, the latter detecting absorption of the field. Alternatively, the transponder uses absorbed energy to re-emit a strong response signal to the transmitter / receiver E.
Other achievements of adaptation / disadaptation 2o of impedance ordered can be envisaged without depart from the scope of the present invention. Use a quarter wave line, a transformer impedance ... are e: r ~ ample two of the multiples means of adaptation and / or impedance mismatching available to the specialist.
More generally, other means of disadaptation and adapting to field or signals well known to specialists may be considered in an equivalent way.
3o In the following description, we will use generally the expression "adapted transponder" for designate a transponder whose antenna is in a adapted state, one of the previous, t means of adaptation to the transmission of the field or radio signals being set artwork.
Otherwise, we will speak of a transponder mismatched.
The operation of a set of transponders as well as transmission protocols from a system to transponders according to the invention will now be described, which will illustrate other advantages of the invention.
Transponders at rest, i.e. outside of the radio field B, are preferably in a state mismatched.
So when multiple transponders enter simultaneously in field B, each transponder can go into a suitable state and be detected or interrogated by the transmitter / receiver without disturbance by the other mismatched transponders.
In the mismatched state, it is expected that a transponder receives a little field and therefore energy.
2o The transponder thus has the minimum energy necessary to pass from the maladjusted state to the state adapted by actuating for example the switch I, I1, I2.
This condition is easily met because a antenna tuned to a slightly distant frequency of the transmitted field frequency always receives weakly the radio field and its energy. Likewise a impedance mismatch between the antenna and the circuit electronic always allows low transmission of 3o radio signal and therefore energy.
A first detection / interrogation protocol provides that a transponder enters a suitable state of randomly. If the time interval during which the transponder remains in the adapted state is reduced, the probability for two transponders are adapted at the same time is low. We obtain thus advantageously good detection or good transponder response.
5 The steps in the reading protocol are therefore following.
- the transponders are: initially unsuitable.
- among the transponders present in the field, therefore receiving a little energy, a transponder passes io in the randomly adapted state.
- radio signals are exchanged between this suitable transponder and transmitter / receiver E.
- this transponder becomes mismatched again.
A second protocol consists in providing that a 15 transponder enters the adapted state after a signal adequate control of the transmitter E. eur The transponders can thus be activated on tour in turn, a single transponder being adapted at a time given.
2o It can also be provided that the transponders are all in a suitable state when they are at rest.
In this third protocol, the transmitter can then send an ordonn <~ nt command to a transponder go into a mismatched state. The issuer may thus deactivate all the transponders in its field except one. The step of signal transmission between the transmitter and this selected transponder can then be unwind.
After this exchange, the transmitter activates another 3o transponder.
And so on until the transmitter has interrogated each transponder in its field.

This command by the transmitter is preferably combined with a known code identification protocol a bit-by-bit transponder.
We can still use a S detection / interrogation in which the protocol begins from an initialization signal from the transmitter / receiver, each transponder re-transmitting a recognition signal after a period of time corresponding to an element of its code l0 identification.
Such protocols and systems of detection / interrogation are for example described in EP-B-0 495 708, in the name of the applicant, the description of which is incorporated herein.
The invention also provides a detection system and / or interrogation of transponders comprising a radio transmitter and receiver and a plurality of transponders as described above likely to appear in his field.
FIG. 2 illustrates an example of such a system of detection. We thus see a set of three P0, P1, P2 transponders and an E transceiver allowing them to be detected and / or interrogated.
The two transponders P1 and P2 are found in 25 the boundaries, shown in dotted lines, of the radio field B emitted by transmitter E.
Each transponder has an antenna, shown for example in Figure 2 as resonant circuit with inductance, capacitance, load and 30 electronic circuit.
Preferably this antenna is used to receive the radio field B and its energy as well as signals radio transmitted by modulating the B field. In this case, the field and the signals have the same so-called frequency frequency determined fe.
We can also foresee that the field and the signals have distinct frequencies, the antenna of the transponder receiving frequency signals determined from transmitter E and the circuit transponder electronics receiving on the other hand field B of any frequency and its energy.
We can still predict that the antenna of the l0 transponder transmits signals of determined frequency intended for receiver E, an electronic circuit of the transponder separately receiving field B of any frequency.
Each transponder therefore has an antenna capable of transmitting: radio frequency signals determined between transmitter and receiver E and transponder.
When the transponders P1, P2 according to the invention are in field B, the transponder P1 can be mismatched or out of tune. This avoids any disruption of a transmission between the transceiver E and the transponder P2 by the other transponder P1 present in the radio field B.
Similarly, at another stage of the transmission, the P2 transponder can be mismatched or detuned to avoid disruption of a transmission between transmitter: eur / receiver E and P1 transponder.
To go to this other step, it is preferable 3o to provide that the sender / receiver: sender sends an order selection to a transpcrndeur of all transponders present. This control signal can be for example the identification code: ication of a transponder.
The transponder concerned then preferably passes in a state selected after this command from of the radio transmitter / receiver.
Finally, the invention provides that the system of detection / interrogation includes a transmitter with a frequency sweep over a reduced range.
This arrangement is advantageous when uses transponders whose antenna can be detuned in frequency. Thus a transponder which would not have been detected or read during the phase Interrogation Io can be detected during this phase frequency sweep.
According to the previous example, if the frequency determinate fe, substantially equal to the frequency fa on which matches the antenna of the adapted transponder, is 130 kHz, the transceiver may include frequency sweep over the 130 kHz - 160 range kHz. The frequency range then contains the frequency fa of resonance of the adapted antennas and the frequency resonance fd of mismatched antennas.
2o The operating mode of the transponders described previously provides that the transmitter generates a field single radio having the determined frequency, this field possibly transmitting information to a transponder as an interrogation signal or initialization in the form of radio signals. This signal radio is preferably a simple field modulation radio.
In general, the invention can be implemented works with a transponder including a 3o energy reception distinct from the reception system radio signals.
Also in previous applications we only mentioned the fact that the transponder antenna was mismatched for avoid absorbing the radio field or signal the transmitter.
But more generally, the signals of response sent by one of the transponders to S the transmitter / receiver can also be absorbed by neighboring transponders :. This effect is all the more more annoying than the transpander generally has of very low energy for such a retransmission.
It is therefore advantageous to provide that the other lo transponders are mismatched to avoid absorbing the response signal re-emitted by a transponder.
In general therefore, the invention provides for a transponder likely to be presented in a field radioelectric, the transponder comprising a circuit 15 electronics capable of absorbing and restoring the energy provided by a te: l field, and an antenna capable of transmitting radio signals from determined frequency, mainly characterized by that the antenna includes means for adapting to 20 transmission of signals of determined frequency and means of adaptation to the transmission of signals of the determined frequency, 7.es means of adaptation of the antenna being used when the transponder is in a separate state, the means of 25 antenna mismatch being implemented when the transponder is in an unselected state of so that the unselected transponder has a limited absorption of radio signals from frequency determined.
3o The invention can be advantageously implemented works in a transmission system comprising a reader or detector for reading information on labels or to detect objects containing such labels. This system applies particularly in commercial applications, for example example for product traceability or tracking stock, labels with price information, identification, or quantity of products 5 correspondents.
Other applications and variations of exhibited achievement are within the reach of man the profession without departing from the scope of the present invention.

Claims (12)

1. Transponder (P1) likely to be presented in a radioelectric field (B), the transponder (P1) comprising an electronic circuit (D) capable to absorb and restore the energy supplied by a such a field, and an antenna (L, C1, C2) capable of transmit frequency radio signals determined, characterized in that the antenna comprises means of mismatching to a transmission of determined frequency signals and means adaptation to the transmission of signals from the determined frequency, the means of adaptation of the antenna being implemented when the transponder is in a selected state, the means to antenna mismatch being implemented when the transponder is in an unselected state of so that the unselected transponder (P1) has limited absorption of radio signals from determined frequency. 2. Transponder according to claim 1, likely to be presented in a field fixed-frequency radio frequency, the transponder (P1) comprising an antenna (L, C1, C2) capable of receiving the radioelectric field (B) of determined frequency and an electronic circuit (D) coupled to the antenna (L, C1, C2), the absorbing circuit and restoring the energy supplied by the field received by the antenna, characterized in that the transponder comprises mismatching means (V1, R) at a reception of a radio frequency field determined and adaptation means (V1,I1,R,I) to the reception of the radioelectric field of the frequency determined, the adaptation means being implemented when the transponder is in a selected state, the mismatching means being implemented when the transponder is in an unselected state so to limit the absorption of energy and/or field (B') by the transponder (P1) in the unselected state. 3. Transponder according to one of the claims preceding characterized in that the means (V1) allow the antenna to be tuned according to the determined radio frequency, and in that the mismatching means (V1, I1) make it possible to detune the antenna with respect to the frequency determined radio frequency. 4. Transponder according to one of the claims preceding characterized in that the means adaptation (V1) and/or mismatching (V1, I1) consist of modifying a capacity value (C1+C2+V1), the antenna having an inductance value (L), so that the tuning frequency of the antenna is modified with respect to the determined frequency of the radio signals. 5. Transponder according to one of the claims preceding characterized in that the means adaptation (R) and/or misadaptation (R, I) make it possible to modify the impedance adaptation between the antenna and the electronic circuit. 6. Transponder according to one of the claims preceding characterized in that the means of adaptation and/or disadaptation consist in change a load and/or resistance value (R) in order to modify the impedance value of the antenna (L,C1,C2) and/or circuit impedance value electronic (D). 7. Set of transponders according to one of preceding claims characterized in that when two transponders (P1, P2) are presented in a radioelectric field, each (P2) of the transponders receives the radio frequency field and/or signals from frequency determined what flood is the position of a transponder (P2) relative to the other transponder (P1), each transponder comprising the means of adaptation and disadaptation to avoid radio interference caused by the proximity to the other transponder. 8. Set of transponders according to one of preceding claims characterized in that the transponders go into a selected state in turn turn, a single transponder or a small number of transponders being in a selected state at a moment of time. 9. Set of transponders according to one of preceding claims, characterized in that a transponder goes into an unselected state for a duration of time corresponding to an inhibition delay of the transponder. 10. System for detection and/or interrogation of transponders comprising a transmitter/receiver radio (E) and a plurality of transponders (P1,P2) likely to occur in a field radio frequency (B) emitted by the transmitter/receiver (E), each transponder comprising an antenna (L, C) capable of transmitting frequency signals determined between the transmitter/receiver (E) and the transponder, the system being characterized in that it contains transponders according to one of the previous claims in order to avoid disturbance of a transmission between the transmitter/receiver and a transponder given by the other transponders appearing in the field radioelectric. 11. System according to claim 10, characterized in that the transmitter/receiver radioelectric comprises a frequency sweep in a range around the determined frequency so that a transponder in an unselected state absorbs energy provided by the field emitted by the transmitter/receiver and enters a selected state so that the transponder is detected and/or interrogates. 12. System according to claim 10 or claim 11 characterized in that a transponder enters a selected state after a command from the radio transmitter/receiver.